The present invention relates to suppression of corrosion that occurs in water tubes or the like of boilers. More specifically, the invention relates to a boiler operating method in which the concentration of sulfate ions SO.sub.4.sup.2- in the boiler water is regulated. The invention also relates to a method for injecting a sulfite base oxygen scavenger which is added to water supplied to the boiler.
Generally, during the operation of a boiler, the pH value of the water within the water tubes, or of the boiler water, increases with generation of hydroxide ions due to thermal decomposition of bicarbonate ions contained in the feed water and with concentration of the boiler water due to evaporation. In this connection, in once-through boilers, which involve no circulation of water in the boiler body, the degree of concentration of boiler water between upper part and lower part of the water tubes differs. As a result, the pH of boiler water increases considerably in the upper part of the water tubes, but does not increase so much in the lower part of the water tubes. This results in a water quality of nearly the same pH value as the feed water. Indeed carbon steel is widely used to make the water tubes in boilers, but carbon steel is prone to corrosion in neutral water containing oxygen. Thus, the water management for boilers involves control processes to remove dissolved oxygen in the feed water and raise the pH of the standards for boiler water. According to the water quality management in JIS (Japanese Industrial Standards), the pH of boiler water should be controlled to be around 11 to 11.8. If this level is regarded as a proper pH level, then the pH in lower part of the water tubes is lower than the proper value. Accordingly, the once-through boiler is prone to corrosion in lower part of the water tubes. To prevent this corrosion, it is conventional to add agents to the feed water for the boiler. These agents are classified roughly into two kinds depending on components contained in the agents. One is called filming type agents, which are agents that form an anticorrosion film on the surface of the water-tube inner wall. The other is called oxygen scavenging type agents, which remove dissolved oxygen in the feed water. (a cause of corrosion), to prevent corrosion.
In past literature concerning water management of boilers, dissolved oxygen, low pH (operation of boilers at pH levels lower than proper) and harmful ions (e.g., chloride ions Cl.sup.-, sulfate ions SO.sub.4.sup.2-) have been mentioned as principal causes of corrosion. However, the effect of these factors on corrosion was examined no more than only qualitatively, the case being unclear. It was also unclear whether the filming type agent or the oxygen scavenging type agent is more effective to prevent corrosion.
Thus, the inventors have experimented to find ways to effectively reduce corrosion of boilers. As a result, it was found that the anticorrosion effect of filming type agents is, in general, largely affected by pH, and sufficient anticorrosion effect could not be expected in a low pH region, i.e., pH=7-9. That is, the anticorrosion effect is insufficient in lower part of the water tubes in the once-through boiler. On the other hand, as a result of making similar corrosion experiments with the oxygen scavenging type agents, such as ascorbic acid, erythorbic acid, MEKO (methyl ethyl ketoxime), hydrazine and sulfite as an oxygen scavenger, it was found that anticorrosion properties do not depend so much on pH (unlike the filming type agents). The oxygen scavenging type agents exhibit a superior anticorrosion effect even with water quality in lower part of the water tubes in the once-through boiler (in a low pH region). Among these oxygen scavengers, sulfite proved to have the highest anticorrosion effect. However sulfite also proved to have a potential defect in that sulfate ions SO.sub.4.sup.2- are generated after sulfite ions SO.sub.3.sup.2- have reacted with dissolved oxygen in the water. Sulfate ions SO.sub.4.sup.2- destroy the anticorrosion film generated on the surface of carbon steel in the water, accelerating corrosion. As the boiler water is concentrated with evaporation, the sulfate ions SO.sub.4.sup.2- are also concentrated so that their increased concentration causes a considerable increase in corrosion. The present invention, having been accomplished in view of these and other problems, has an object of effectively suppressing corrosion by using a sulfite base oxygen scavenger.